1. Field of the Invention
This invention relates generally to information reproducing systems capable of accessing a desired data or information record track on a disc-shape information carrier, and more particularly, to a track jump servo system for disc players having servos for positioning a transducer means rapidly and accurately to a desired record track location on a disc surface.
2. Background of the Invention
Digital record discs for recording information signals such as digitized audio signals, video or image signals have become quite common as have record disc reproducing apparatus for reproducing the recorded information data. Further, some digital record discs, e.g., so-called CDs (Compact Discs) have recently become used as ROM (Read Only Memory) devices for computers, called CD-ROMs. On such a digital record disc, digital data intended for high density recording are recorded in the form of a spiral bit string on one side thereof and reproduced through reading the bit string by means of transducer means such as an optical pick-up device using laser beams. The optical pick-up device of the record disc reproducing apparatus follows the bit string by means of a tracking control system. CD players or video disc players are further used to process the read-out signal in a predetermined manner so as to convert the signal into a replica of the original analog signal which was recorded, i.e., an audio or video signal as an output.
As is well known, address information may also be recorded on the record disc which may be detected for search and track jump operation so that reproduction at a desired track location can be easily and quickly achieved. In particular, a selected address corresponding to the desired track location is preset, and the pick-up device is rapidly moved across or traverses to the track until the desired track corresponding to the selected address is reached. During such searching, a distance that the pick-up device should jump or traverse is calculated by, for example, a computer in accordance with the address of the track at which the pick-up device is presently located and the address of the desired track. Then the pick-up device is moved by the distance radially across the disc by applying a predetermined, fixed voltage to a driver such as a voice coil, moving coil or linear motor. The fixed voltage causes the pick-up device to continuously accelerate until the voltage ceases.
In a short distance search operation of only a few tracks, the pick-up device can jump to the desired track relatively accurately. In a long distance search operation, however, to a track located dozens of tracks away from the track at which the pick-up device is presently located, the pick-up device does not become accurately placed on the desired track for a long time. During a long distance search operation, the actual distance that the pick-up device has moved is apt to differ from the desired distance calculated by the computer. As the distance the pick-up device should jump, or traverse becomes longer, a track at which the pick-up device has been placed at the end of the jump may differ more from the desired track due to mechanical loss or non-uniformity in the mechanism for transferring the pick-up device, inertia of the pick-up device, temperature conditions, the unpredictable speed when the desired track is reached and the like.
Therefore, the search operation includes repeated track jumps so that the pick-up device gradually approaches the desired track location. After every track jump in the search operation, the track address is checked to determine whether a track where the pick-up device has been placed after the track jump coincides with the desired track or not.
As a result, it is difficult to ensure that the pick-up device jumps over or traverses rapidly and accurately the record tracks to a desired track location on the disc.